1. Field of the Invention
The present invention relates to a scanning optical system for scanning a laser beam on a scanning surface to form a pattern thereon, and particularly to a system to be able to reduce a curvature of field.
2. Description of the Prior Art
Japanese Patent Early Laid-Open Publication No. Sho 63-146015 discloses a scanning optical system having an anamorphic optical system. A laser beam emitted from a light source is imaged on reflecting surfaces of the polygon mirror in an auxiliary scanning plane by the anamorphic optical system so as to correct an affection caused by a tilting error of the reflecting surface used as a deflector.
Since the optical system of the Japanese publication cited above has a large positive power in the auxiliary scanning direction in order to temporarily image the light beam on the reflecting surface, a focusing point of the laser beam approaches the f.THETA. lens. In order to correct the curvature of field, a cylinder surface or toric surface having a negative curvature in the auxiliary scanning plane may be used for other lenses.
Also, in an optical system which provides a polygon mirror as a scanning deflector, since a radius of the polygon mirror is made relatively large, the curvature of field is corrected by a change of the deflecting point of the polygon mirror. If the number of the reflecting surfaces is fixed, the amount of the change of the deflecting point is varied by the radius of the polygon mirror.
However, if the negative power in the auxiliary scanning direction is made strong, it is possible indeed to favorably correct the curvature of field, but the wave aberration in the skew direction is deteriorated, thus resulting in a degraded spot shape.
Also, in case of a general scanning optical system such as a system of a laser beam printer, since the laser beam is incident to the polygon mirror from a direction from the optical axis of the scanning lens, a deflecting point on the reflecting surface of the polygon mirror changes asymmetrically about the optical axis. Therefore, a problem occurs such that the curvature of field become asymmetric on the scanning surface and correction can not be made when a lens is symmetric about the optical axis. As the radius becomes larger the curvature of field becomes smaller. Also, as the radius become larger the asymmetry also becomes larger.
In particular, such a problem conspicuously appears in a system having high accuracy and wide scanning extent.
For example, in such an apparatus as laser processing which requires such high accuracy in dot density, for example, 1000 dpi or more, a lens system having an F number of about 1:25.about.1:35 is required in order to reduce the spot diameter to about 30 .mu.m and deterioration of the quality of an image caused by wave aberration in the skew direction becomes significant.
Also, for laser processing devices used for printing posters, there are some devices which have a scanning extent of 600 mm or more.
Since the focal length of the lens system becomes long, it was very difficult to favorably correct the wave front aberration and to correct the curvature of field.